This invention relates generally to an image sensor comprising a charge transfer device such as a charge coupled device (hereinafter referred to as CCD), and more particularly to a solid state image sensor comprising the charge transfer device in which transfer electrodes for the charge transfer device are improved in construction so that the number of effective sensing elements can be altered in accordance with the manner of supplying a driving signal to the transfer electrodes.
Research and development for a solid state image sensor which comprises a charge transfer device, especially, the CCD have progressed very fast and several compact television cameras employing such a solid state image sensor have been proposed for producing not only a monochromatic television signal but also a color television signal. For the color television systems, the NTSC system, the PAL system and the SECAM system have been put into practical service at the present. The NTSC, PAL and SECAM systems are based on different technical concepts, respectively, and each of them have a different number of horizontal scanning lines forming a frame picture. In the NTSC system, one frame period consists of 525 line periods and one frame picture is formed with a video signal obtained in about 485 line periods among the 525 line periods except the line periods in two vertical blanking periods therein, in other words, one frame picture is formed with about 485 horizontal scanning lines. While, in the case of the PAL or SECAM system, one frame period consists of 625 line periods and one frame picture is formed with a video signal obtained in about 570 line periods among the 625 line periods except the line periods in two vertical blanking periods therein, in other words, one frame picture is formed with about 570 horizontal scanning lines.
Each of the previously proposed color television cameras for producing the color television signal according to the NTSC, PAL or SECAM system which employ the solid state image sensor, requires a separate solid state image sensor designed specifically for the NTSC, PAL or SECAM system. The reason of this is that in the solid state image sensor comprising, for example, the CCD, each cell for storing and transferring a signal charge in the CCD forms a sensing element and the number of rows of such sensing elements for performing the charge transfer in the vertical direction determines the number of the horizontal scanning lines forming one frame picture in an output video signal, and further the number of the sensing elements making each of the rows, that is, the number of the sensing elements in the horizontal direction determines the definition in each horizontal scanning line, and therefore it is necessary for obtaining the color television signal according to the NTSC, PAL or SECAM system as an output video signal to use the solid state image sensor having the rows of sensing elements for performing a number of charge transfers in the vertical direction which is the same as the number of the horizontal scanning lines forming one frame picture in the respective system, that is, about 485 rows for the NTSC system or about 570 rows for the PAL or SECAM system. Incidentally, since the image sensing operation is performed field by field in general, the number of the rows of sensor elements arranged in the vertical direction which become operative at the same time corresponds with one half of the number of the horizontal scanning lines forming one frame picture. Further for the additional reason that the ratio of the vertical size to the horizontal size in a picture displayed on a television receiver is generally determined at 3:4, and therefore it is required to select the number of the sensing elements making each of the rows arranged in the vertical direction in response to the number of the rows of sensing elements in order to obtain the definition which will remain constant in the horizontal direction. Consequently, the solid state image sensor used for the PAL or SECAM system is required to have a number of the rows of sensing elements for performing the charge transfer in a vertical direction and the number of the sensing elements which form each of the rows which are greater than the corresponding ones used in the solid state image sensor for the NTSC system, respectively.
However, with a solid state image sensor in which the number of effective sensing elements which are operative to perform the image sensing which can be altered as occasion demands, it will be possible to use it for each system for obtaining the color television signals according to the NTSC,
and SECAM systems and this will result in great convenience. In view of this desire, a novel solid state image sensor is proposed in the present invention.